Room-temperature hyperpolarization of polycrystalline samples with optically polarized triplet electrons: pentacene or nitrogen-vacancy center in diamond?

We demonstrate room-temperature ¹³C hyperpolarization by dynamic nuclear polarization (DNP) using optically polarized triplet electron spins in two polycrystalline systems: pentacene-doped [carboxyl-¹³C] benzoic acid and microdiamonds containing nitrogen-vacancy (NV^-) centers. For both samples, the integrated solid effect (ISE) is used to polarize the ¹³C spin system in magnetic fields of 350-400 mT. In the benzoic acid sample, the ¹³C spin polarization is enhanced by up to 0.12 % through direct electron-to-¹³C polarization transfer without performing dynamic ¹H polarization followed by ¹H-¹³C cross-polarization. In addition, the ISE has been successfully applied to polarize naturally abundant ¹³C spins in a microdiamond sample to 0.01 %. To characterize the buildup of the ¹³C polarization, we discuss the efficiencies of direct polarization transfer between the electron and ¹³C spins as well as that of ¹³C-¹³C spin diffusion, examining various parameters which are beneficial or detrimental for successful bulk dynamic ¹³C polarization.